1. Field of the Invention
The present invention relates to a silicon-bonded wafer, and the measurement method for measuring the thickness of the upper wafer during or after it was thinned.
2. Description of the Related Background
In the semiconductor wafer bonding technology, there is a technique for bonding two wafers together to form a new wafer, that is, two silicon wafers (with silicon oxide, silicon nitride or nothing on the surface thereof) are adhered together by the Van der Waal's force therein, and then bonded in high temperature. The new wafer, of which the thickness of the upper wafer is thinned to a specific thickness by polishing process, thus it becomes a thin film layer on the new wafer, and a new wafer with a new structure is accomplished. It can be provided for the devices that are in need of the wafer with such structure for the follow-up processes. In the polishing process, it is a very important procedure that the way to measure the thickness after the upper wafer had been thinned and then to sort the wafers according to the specifications thereof.
In the processes for manufacturing MEMS devices, diaphragm are applied for many purposes. The properties of the MEMS devices, such as the pressure sensors and the accelerometers, are concerned with the thickness of the diaphragm. After the process for bonding the two silicon wafers, one of the wafers is polished and thinned, and a diaphragm is formed. With respect to the measurement of the wafer thickness, there are two types of methods for the measurement as follows:
The first one is a measurement method applying a polisher with a contact altimeter. The thickness of the wafer is firstly measured by the altimeter, then the thickness to be removed is calculated, after that, the polishing time of the polisher is set according to the used remove rate of the polisher. Thus, a wafer with expected thickness is obtained, and then it is measured again by the altimeter for determining the specifications. Such method is applied commonly by the industrial circles. However, while a plurality of wafers are going to be polished, since the wafers thickness are varied from several to tens of μm, and since the variation of the derived stress and the remove rate would influence the results of polishing. On basis of the above reasons, the thickness of the polished wafers are varied from several tens of μm. In order to obtain a wafer with more precise thickness, a generally used method is to sort the wafers by measuring the thickness. However, as the two wafers are bonded, the thickness of the upper wafer that thinned is hard to be measured individually.
The second one is a non-contact, instantaneous optical measurement method. It applies a phenomenon that when the surface of the to-be-measured material is illuminated by a light source, a part of the light would be reflected by the first material (namely received reflective signal 1), the another part of the light would be transmitted through the first material. Once the light is in contact with the second material that different from the first material, reflection and transmission is occurred again. As the reflected light is transmitted out of the to-be-measured materials (namely received reflective signal 2), two parallel reflective optical signals are received by the sensor, and the thickness of the to-be-measured materials can be figured out immediately. This method can be applied to chemical mechanical polishing, especially for the layer of material in order to improve surface smoothness. However, it is not a suitable method for independently measuring the reduced thickness of a single layer from the two bonded wafers, which are made of the same material of silicon.
Accordingly, the present invention provides a measurement method for measuring the thickness of a single layer from the silicon wafers that are thinned after bonded together. Since the silicon lattice having the property of anisotropic etching (as shown in FIG. 1, in a KOH solution, the etching rate of silicon (100) is much greater than that of silicon (111)). Several inspection patterns with varied size is generated on one side of the wafer, and then after it is proceeded with silicon bulk micromachining for a specific time, several V-grooves with varied depth for inspection is formed. The V-groove inspection pattern is shown in FIG. 1 and the included angle between the silicon (100) and the silicon (111) is 54.7°. The actual thickness of the upper wafer after polished can be calculated by measuring the size of the gaps of the polished V-grooves, and then it is sorted according to the remaining thickness. The MEMS micro-sensor devices, such as silicon-based piezoresistive pressure sensors or accelerometers etc., of which the output signals is in concerned with the thickness of the diaphragm, by applying this method, the polished upper wafer can be sorted by thickness precisely. Consequently, the micromachined devices with similar magnitude of output signals can be accomplished.
3. Description of the Prior Art
Some measurements for the polished thickness of a wafer have been disclosed before, such as the mechanical contacting measurement or the optical non-contacting measurement, and they are now described as below:                (1) In the thickness measurement in a rear wafer polishing process of the Taiwan Pat. No. 359,746 disclosed by P. W. Tsai et al, wherein a mechanical measurement is applied. The total thickness of wafer can be measured by a diamond probe 21 contacting to the wafer 20, then the obtained signals are transmitted to the signal detector inside the measuring box 23 by the measuring rod 22. Please refer to FIG. 2 and the reference material (1) for details.        (2) In the chemical mechanical polishing process of the U.S. Pat. No. 6,301,006 disclosed by Trung T. Doan, wherein the non-contacting instantaneous optical measurement is applied for measuring the thickness. The wafer 30 is put in the wafer carrier 31. Then, the measuring surfaces on the wafer carrier is detected by the detector above in order to measure the parameters upon the wafer thickness and the remove rate during the wafer polished. Please refer to FIG. 3 and the reference material (2) for details.        (3) In the chemical mechanical polish of the U.S. Pat. No. 6,071,177 disclosed by C. L. Lin et al, wherein the thickness is measured by non-contacting instantaneous optical measurement, specially for the silica film thickness control for polishing the silicon wafer. An optical detector module 41 is placed under the light-permeable polishing pad, and then the wafer is rotated around the center of the polishing pad. Once the wafer 40 is passed through the optical detector module 41, the film thickness is measured. Please refer to FIG. 4 and the reference material (3) for details.        
In applying the methods disclosed above, the thickness of an individual layer of the two wafers, which are bonded together and polished, cannot be measured effectively. Therefore, a new method for measuring the thickness of the polished wafer is required for overcoming the properties variations of the sensor devices caused by the thickness variations of the upper silicon wafer.